This invention relates to an amplitude limiter for use in an AM broadcast transmitter and, more particularly, to such an amplitude limiter which is particularly adapted for amplitude modulated (AM) stereophonic transmission wherein energy of the broadcasted AM stereo signal is substantially constrained within an allocated broadcast bandwidth, thereby minimizing energy of the AM stereo signal which might be present outside this allocated bandwidth.
In amplitude modulated (AM) transmission systems, the level, or degree, of the modulating signal should be limited to prevent excessive or overmodulation of the carrier. Such overmodulation, generally in excess of 100%, may result in distortion of the information signal which is recovered at a transmitter and, also, may result in the generation of second order and higher harmonics, generally referred to as higher order spectra. Typically, for radio broadcast purposes, a transmitted or broadcasted signal is limited to an allocated bandwidth. In the United States, this bandwidth is limited to 30 KHz symmetrically disposed with respect to the carrier frequency. Hence, with a carrier frequency of .omega..sub.c, energy of the AM signal having this carrier must be limited so as to have little, if any, energy outside the bandwidth of .omega..sub.c .+-.15 KHz.
Generally, the information signal which is used to amplitude modulate the carrier in most radio broadcast systems is an audio signal. As is known, the amplitude of an audio signal, which represents sound such as voice and music, exhibits a wide dynamic range. In the absence of any limiting circuitry, it is possible that the audio signal which modulates the carrier may exhibit a sudden large increase in amplitude. As a result, the carrier may be overmodulated, thereby generating harmonics, or higher order spectra, outside the allocated bandwidth. The energy of such higher order spectra will, therefore, be present in the bandwidth which is allocated to another carrier frequency, thereby interfering and distorting the signal which is transmitted by that other carrier.
This problem of overmodulation in relatively simple AM transmission systems has been solved by limiting the modulation level of the audio signals to no more than 100% and, in many instances, the audio signal is limited to a level of 95 to 100%. However, when an AM transmission system is utilized to broadcast compatible stereophonic signals, such a relatively simple amplitude limiter does not fully satisfy the requirement of minimizing energy in the broadcasted signal outside the 30 KHz bandwidth.
The signal transmitted by a compatible AM stereo broadcast system must contain modulated information that can be received and recovered by conventional monaural receivers as well as stereo information which can be recovered by special stereophonic receivers, the latter being operative to reproduce stereophonic sound. Accordingly, a monaural, or "mono" component is used to amplitude modulate the carrier, and the "stereo" component, referred to herein as the submodulation component, is used to modulate either another characteristic of the carrier or an additional subcarrier. Various proposals have been made for a so-called AM-PM AM stereo system, wherein the mono component is used to amplitude modulate the carrier and the submodulation, or stereo, component is used to modulate the phase of that carrier. Such an AM-PM broadcast signal is compatible with monaural receivers because the amplitude modulated mono signal is readily recovered therefrom. Likewise, this AM-PM signal, when detected by special receivers designed therefor, permits the mono component to be recovered as well as the stereo, submodulation component, and these components then may be further processed, as by matrixing, to reproduce stereophonic sound. Other proposals have contemplated amplitude modulation or frequency modulation of a subcarrier by the submodulation, or stereo, component. These other proposals have been known as AM-AM and AM-FM systems.
The mono component of an AM stereo signal is recognized as the (L+R) component; and the submodulation, or stereo, component is recognized as the (L-R) component, wherein L and R represent the left-channel and right-channel signals, respectively, in an audio broadcast system. Overmodulation due to the mono component (L+R) may be avoided by limiting the amplitude of the left-channel signal (L) and the right channel signal (R), respectively. However, even if the sum of the left-channel and right-channel modulating levels is limited in this manner, the level of the submodulation component (L-R) still may be so great as to result in harmonics in the broadcasted signal, which harmonics lie outside the bandwidth allocated to the broadcast frequency.
One type of limiter which has been proposed for AM transmission includes a variable gain amplifier for amplifying the mono component with a gain that is controlled as a function of the level of that amplified component. The amplified mono component, that is, the output of the variable gain amplifier, is rectified and compared to a threshold level which represents a modulation degree of 95% to 100%. If the output of the amplifier exceeds this threshold level, which otherwise would result in overmodulation of the carrier, the gain of the amplifier is reduced. To use this limiter in an AM stereo transmission system, separate controllable variable gain amplifiers may be used, one to control the amplified level of the mono component (L+R) and the other to control the amplified level of the submodulation component (L-R). The respective threshold levels which are used to control the gains of these amplifiers may be established such that the levels of the mono and submodulation components, together, do not exceed 100%.
The present invention proceeds on the recognition that secondary and higher harmonics of the AM stereo signal may be produced even if the submodulation component in the frequency range of from one-fourth to one-half the allocated bandwidth (e. g. the submodulation component having frequencies in the range of 7.5 KHz to 15 KHz) has a level that is substantially less than 100%. That is, substantial amounts of energy will be present in an adjacent allocated broadcast bandwidth if the level of the audio signals in this frequency range exceed 50%. However, typical amplitude limiters, such as that described above, do not perform a frequency-selective limiting operation and, thus, undesired higher order spectra nevertheless may be produced even when the aforementioned amplitude limiter limits the combined modulation level of the mono and submodulation components to 100%.